[RFC PATCH 59/86] treewide: rcu: remove cond_resched()

[Ankur Arora]

All the cond_resched() calls in the RCU interfaces here are to
drive preemption once it has reported a potentially quiescent
state, or to exit the grace period. With PREEMPTION=y that should
happen implicitly.

So we can remove these.

[1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@xxxxxxxxxx/

Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> 
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> 
Cc: Ingo Molnar <mingo@xxxxxxxxxx> 
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> 
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> 
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> 
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 include/linux/rcupdate.h | 6 ++----
 include/linux/sched.h    | 7 ++++++-
 kernel/hung_task.c       | 6 +++---
 kernel/rcu/tasks.h       | 5 +----
 4 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 7246ee602b0b..58f8c7faaa52 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -238,14 +238,12 @@ static inline bool rcu_trace_implies_rcu_gp(void) { return true; }
 /**
  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
  *
- * This macro resembles cond_resched(), except that it is defined to
- * report potential quiescent states to RCU-tasks even if the cond_resched()
- * machinery were to be shut off, as some advocate for PREEMPTION kernels.
+ * This macro resembles cond_resched(), in that it reports potential
+ * quiescent states to RCU-tasks.
  */
 #define cond_resched_tasks_rcu_qs() \
 do { \
 	rcu_tasks_qs(current, false); \
-	cond_resched(); \
 } while (0)
 
 /*
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 199f8f7211f2..bae6eed534dd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2145,7 +2145,12 @@ static inline void cond_resched_rcu(void)
 {
 #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
 	rcu_read_unlock();
-	cond_resched();
+
+	/*
+	 * Might reschedule here as we exit the RCU read-side
+	 * critical section.
+	 */
+
 	rcu_read_lock();
 #endif
 }
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 9a24574988d2..4bdfad08a2e8 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -153,8 +153,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
  * To avoid extending the RCU grace period for an unbounded amount of time,
  * periodically exit the critical section and enter a new one.
  *
- * For preemptible RCU it is sufficient to call rcu_read_unlock in order
- * to exit the grace period. For classic RCU, a reschedule is required.
+ * Under a preemptive kernel, or with preemptible RCU, it is sufficient to
+ * call rcu_read_unlock in order to exit the grace period.
  */
 static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
 {
@@ -163,7 +163,7 @@ static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
 	get_task_struct(g);
 	get_task_struct(t);
 	rcu_read_unlock();
-	cond_resched();
+
 	rcu_read_lock();
 	can_cont = pid_alive(g) && pid_alive(t);
 	put_task_struct(t);
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 8d65f7d576a3..fa1d9aa31b36 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -541,7 +541,6 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
 		local_bh_disable();
 		rhp->func(rhp);
 		local_bh_enable();
-		cond_resched();
 	}
 	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 	rcu_segcblist_add_len(&rtpcp->cblist, -len);
@@ -974,10 +973,8 @@ static void check_all_holdout_tasks(struct list_head *hop,
 {
 	struct task_struct *t, *t1;
 
-	list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
+	list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list)
 		check_holdout_task(t, needreport, firstreport);
-		cond_resched();
-	}
 }
 
 /* Finish off the Tasks-RCU grace period. */
-- 
2.31.1